Computer systems use main memory that is typically formed with inexpensive and high density dynamic random access memory (DRAM) chips. When a particular row in a DRAM chip is activated for reading or writing, the word line associated with the row is activated, and the contents of the memory cells along the row are read into a page buffer. Subsequent read and write accesses to memory cells in the row can take place wholly within the page buffer, without accessing the row again. When a data processor later accesses another row in the same memory bank, the memory cells along the row are restored in a precharge operation before the other row can be activated.
Modern DRAM chips typically store one to eight gigabits (Gb) of data using deep sub-micron technology. Because of the high density and small feature size, rows of the memory are so physically close to other rows that the activation of a particular row can upset data stored in adjacent rows by changing the charge on the memory cell capacitors. Typically these upsets are harmless because the memory cells are refreshed periodically. However occasionally some memory access patterns cause certain rows to be activated and precharged so many times before the next refresh cycle that the memory cells in adjacent rows become corrupted and reverse logic state. After being corrupted, the original data is lost and cannot be restored in subsequent refresh cycles.
One known technique to address the data upset problem is known as targeted row refresh (TRR). In order to ensure that a DRAM row is not activated too many times within a refresh period, a memory controller places the DRAM into a TRR mode by setting certain mode register bits. The controller then issues successive activate and precharge commands to the target row as well as the two physically adjacent rows. Once TRR mode is enabled, no other mode register commands are allowed until the TRR mode is completed. TRR mode is self-clearing and the mode register bit is set after the completion of TRR mode. While TRR allows the memory controller to avoid excessive activates to a certain row within a certain time period, it is entered by setting the mode register, which requires a substantial amount of time since all banks must be in the idle state before the controller can issue a Mode Register Set command.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.